form using two placeholders and the visible function error Invalid Post Back

I have a page asc.aspx this page has two placeholders (frmasc_search & frmasc_results) the first is visible on page load and the other is not. When I press a button I want the first form to change to invisible and the second to be visible. However when I do this I am getting an error for Invalid Post Back. What do I need to do. I have included the full text of the error and the script I am running below.

---- Error Message ----Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.ArgumentException: Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.Source Error: An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [ArgumentException: Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.] System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument) +159 System.Web.UI.Control.ValidateEvent(String uniqueID, String eventArgument) +108 System.Web.UI.WebControls.TextBox.LoadPostData(String postDataKey, NameValueCollection postCollection) +36 System.Web.UI.WebControls.TextBox.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) +12 System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) +353 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1194---- Script ---- <script runat="server">Sub Page_Load (sender As Object, e As EventArgs)ph_asc_search.visible = trueph_asc_results.visible = falseEnd SubSub brtnsearch_Click(sender As Object, e As EventArgs)ph_asc_search.visible = falseph_asc_results.visible = trueEnd Sub</script>